With recent remarkable widespread use of frozen food and technical innovation in the freezing field, frozen cooked-rice has also been commercialized which is prepared by freezing cooked rice, is preserved in a frozen state, and is thawed and heated, if necessary, when eaten. In the past, frozen cooked-rice was mainly in the form of a block enclosed in a container or bag. The frozen cooked-rice in the block form however had such disadvantages that a long time was required for thawing because heat was hard to penetrate the frozen block, and that the taste was spoiled because the thawed cooked-rice was less loosened and the individual grains of rice were likely to break down. Techniques for individually freezing cooked rice have been developed since then as an approach for overcoming those disadvantages, and are generally called IQF (individuated quick freezing). Various proposals for the techniques for individually freezing cooked rice have been made so far.
Typical methods practiced in individuated freezing apparatus for cooked rice on industrial scale are mainly divided into three types below (as described in "Rice Cooking Techniques and Their Applications", edited by Hiroaki Horigome, Kogyo Gijutsukai K. K., pp. 209-225 (1990)).
(1) A method of freezing cooked rice in liquefied coolant gas, such as liquid nitrogen, and dry ice while agitating the cooked rice.
This method can provide good products in both points of individuation and freezing, but it is disadvantageous in pushing up the cost.
(2) A method of freezing cooked rice in the form of masses of cooked rice and disintegrating the masses.
This method is disadvantageous in that the individual grains of rice are likely to break down at the same time as when the masses of cooked rice are disintegrated individually.
(3) A method of freezing cooked rice while loosening it by a combination of the so-called air blasting, in which cooled air is blown to cooked rice, and a rotary comb, or a combination of vibration and a rotary comb.
The last method has been mainly used in the industrial field and various improvements have been proposed in recent years.
Generally, conditions for individuated freezing of cooked rice are set such that the temperature in a freezing apparatus is around -40 to -45.degree. C., the freezing time is about 3 to 5 minutes, and the freezing finish temperature is under -20.degree. C.
As the above method (3) for individually freezing cooked in air blasting, there are proposed, e.g., a process for individually freezing cooked rice by repeating a combination of carrying under vibration and falling ["Freezing and Individuating Apparatus for Cooked Rice" (Japanese Unexamined Patent Publication No. Sho 57-99148)], and a process for individually quickly freezing cooked rice by loosening the cooked rice while it is moved in a meshed rotary drum ["Drum Apparatus for Individuating Cooked Rice" (Japanese Unexamined Patent Publication No. Sho 63-44857)]. However, the above processes are disadvantageous in that the apparatus have complicated and special constructions.
A process for individually freezing cooked rice by using a belt conveyor as feed means and rotating a rotary comb in air blasting is advantageous in that the belt conveyor used as the feed means has versatility and is easy to carry out cleaning and maintenance service.
(3-1) "Freezing and Individuating Method and Apparatus for Cooked Rice" (Japanese Unexamined Patent Publication No. Sho 60-227644)
Above a conveyor for carrying cooked rice placed thereon in the form of a layer while cooling and freezing the cooked rice, there is provided a fixed comb combined with a rotary comb such that comb teeth of the fixed comb mesh with comb teeth of the rotary comb. With this combination, the cooked rice is individuated by actions of impact force and shearing force caused upon meshing of the rotary comb and the fixed comb. According to the disclosure of the Publication, it is particularly possible to promote individuation of the cooked rice into separate grains.
However, this method and apparatus have such a drawback that if the cooked rice is individuated only by actions of impact force and shearing force caused upon meshing of the rotary comb and the fixed comb as disclosed in the Publication, the grains of rice would be subject to an excessive load and would be more likely to break down.
(3-2) "Loosening and Conveying Apparatus for Cooked Rice" (Japanese Unexamined Patent Publication No. Hei 2-9352)
This conveying apparatus comprises a primary freezing conveyor for carrying cooked rice, and a secondary freezing conveyor disposed downstream of the primary freezing conveyor to run from a position lower than it by a step in overlapping relation. A rotary loosening device is provided at a position where the cooked rice falls down from the primary freezing conveyor to the secondary freezing conveyor. According to the disclosure of the Publication, palatable frozen cooked-rice can be produced.
However, this conveying apparatus has such a drawback that the cooked rice cannot be individuated sufficiently only by a combination of natural falling of the cooked rice and the rotary loosening device as disclosed in the Publication.
(3-3) "Individuated Freezing of Cooked Rice" (Japanese Unexamined Patent Publication No. Hei 8-103232).
Cold air is blown to cooked rice from a plurality of jet nozzles disposed above and below a conveyor's carrying surface in opposed relation so that water on the surfaces of individual grains of rice is quickly frozen without freezing the cores of individual grains of rice. Resulting frozen blocks of the cooked rice are then disintegrated for individuation. According to the disclosure of the Publication, the grains of rice are less likely to break down.
However, providing a plurality of jet nozzles above and below the conveyor's carrying surface in opposed relation, as disclosed in the Publication, is disadvantageous in making the apparatus construction complicated.
(3-4) "Freezing and Individuating Method and Apparatus for Cooked Rice" (Japanese Unexamined Patent Publication No. Hei 8-205796).
Frozen blocks of cooked rice are divided into small pieces, and the small pieces are sent to a plurality of rotary combs for individuation of the frozen cooked-rice. The rotary combs are provided side by side in the carrying direction and are rotated at progressively increasing rotational speeds. According to the disclosure of the Publication, the frozen cooked-rice can be smoothly disintegrated and individuated without breaking down the individual grains of rice.
However, providing a plurality of rotary combs side by side in the direction of running of the conveyor, as disclosed in the Publication, is disadvantageous in making the apparatus construction complicated.